1. Field of the Invention
The present invention relates to an automatic power transmitting system for an automotive vehicle, which uses an automatically operated constant-mesh transmission with synchromesh devices.
2. Discussion of the Prior Art
As an automatic power transmitting system for motor vehicles, there is known a system which incorporates a transmission wherein a hydraulically operated frictional coupling device is provided to bring component elements of a planetary gear mechanism into mutual engagement with each other, or into engagement with a housing body, thereby enabling the transmission to achieve automatic shifts to suitable drive positions of the transmission. In this type of automatic power transmitting system, the input shaft of the transmission is generally connected to the crankshaft of the engine of the vehicle via a fluid coupling clutch. Because of a slip in the fluid coupling clutch, a vehicle employing an automatic transmission of the type indicated above inevitably suffers from a low level of fuel economy, as compared with a vehicle using a conventional manual transmission. It is also known in the art to provide a fluid coupling with a lock-up device. However, the operational application of such a lock-up clutch is usually limited to some of the gear positions of the transmission. Thus, the use of a lock-up device will not completely overcome the drawback of the above-indicated type of automatic transmitting system, from the standpoint of the fuel economy or specific fuel consumption of the vehicle.
To solve the above drawback, it is considered to use an automatic power transmitting system incorporating an assembly of an electromagnetic powder clutch connected to the engine of a vehicle, and a constant-mesh transmission whose gear shifting operations are automatically effected by a suitable actuator. The engaging and disengaging actions of the powder clutch are controlled in response to the operation of the constant-mesh transmission. This automatic power transmitting system appears to provide at least the same level of fuel economy as a manually operated transmission system. However, the power transmitting system in question has other drawbacks. Described more specifically, an electromagnetic powder clutch usually has a driving rotor in which a relatively large-sized annular coil is embedded, and a driven rotor which cooperates with the driving rotor to define a gap for accommodating a magnetic powder, and to produce a magnetic path for connecting the two rotors through the magnetic powder. The driven rotor is commonly formed from a relatively thick-walled member, for assuring a sufficient area of the magnetic path to be produced. Accordingly, the moment of inertia of the driven rotor and consequently that of the input shaft of the constant-mesh transmission coupled to the driven rotor tend to be relatively high. This indicates a large amount of rotational energy consumption in synchromesh devices that are provided for synchronizing the speeds of two rotating members of the constant-mesh transmission upon shifting actions to select the desired drive position. Therefore, the synchronizing actions or shifting actions take a comparatively long time, resulting in deteriorating the drivability of the vehicle. On the other hand, an attempt to reduce the shifting time will lead to lowering the durability of the synchromesh devices.